1. Field of the Invention
The present invention relates to the field of impact printers and in particular, relates to hammer face design for impact hammers.
2. Prior Art
It is a well established phenomena in the digital printer art that print smear between the hard copy printout and type font, such as on a rotating print band, is created by the duration and nature of impact between the hammer and font type. Generally, the greater the effective weight of the hammer, the greater the contact time between the hammer and the type font wherein the type font is pressed against an inked ribbon next to the paper printout. As a result of the finite contact time, a degree of print smear occurs which in some cases can be excessive and destroy or impair the readability of the printed image. Print smear is particularly noticeable as font size decreases and font velocity increases.
Generally, prior art hammer designs, such as shown in FIG. 1, attempted to minimize the effective weight of the hammer and thereby create a shorter contact time on the tip surface 10 of hammer 12 with the font during impact. This was accomplished, as shown in FIG. 1, by reducing the size and mass of body 14 of hammer 12, leaving portions of tip surface 10 cantilevered at a distance from center line 16 of hammer 12. This was particularly the case where, in a multiple spanning hammer, such as a double spanning hammer, the odd and even print positions 18 and 20 may be one-half the total distance of tip surface 10 from center line 16.
FIG. 2A represents a prior art hammer having a flat tip surface 10 just before the moment of impact with a font type 22. FIG. 2B illustrates the impact between surface 10 and a type character "H", thereby imprinting upon a hard copy surface 24 the inked image of the letter "H". If hammer 12 were perfectly rigid, the smear across the letter "H" would be even and a clear character would be produced as shown in FIG. 2B. However, the impact force of hammer 12 causes even edge 26 to be bent away from font type 22 as shown in FIG. 2C in exaggerated form. That portion of the font character furthest from even edge 26 is more smeared than the nearer portions of the font character as shown in FIG. 2C. The degree of smear increases as font speed and impact force increases.
FIG. 2D illustrates the end of impact wherein the printed "H" has a leg 30 substantially thicker than leg 32. Similarly, as shown in FIG. 2E, a font type 22 printed in odd print position 18 will have a mirror image smear such that leg 34 of an "H" will leave a thicker imprint than leg 36.
One of the prior art methods attempted to avoid the smear phenomena illustrated in FIG. 2 by the means shown in FIG. 3. In the prior art embodiment of FIG. 3, hammer 12 was provided with a hammer face configuration such that tip surface 10 was given a longer trailing edge 26 than leading edge 28. The compensatory theory is based on the following observation. As shown in FIG. 4, a frictional stress exerted by the movement of font type 22 against tip surface 10 can be vectorially resolved into a component parallel to deformed surface 40 and a component perpendicular and away therefrom. As a result, the bending stress on even edge 26 is decreased. On the other hand, as shown in FIG. 5, the same frictional force, F, between font type 22 and deformed surface 42 is resolved into a force, Fp, parallel to deformed surface 42 and a perpendicular force, Fn, directed into tip surface 10. As a result, the degree of stress applied to odd edge 28 is greater. Therefore, the amount to which that portion of hammer 12 was cantilevered outward in the proximity of odd print position 18 is decreased relative to the cantilevered projection in the proximity of even print position 20. The object of the modification was to equalize the amount of bending at both even and odd print positions 18 and 20 without reducing or eliminating it. However, unequal print smear still occurs within any given character imprint. However, as it may be readily appreciated, it is extremely difficult to design the resiliency or deformability of a print hammer across a significant range of impact forces and font velocities such that deformation is equalized in a broad range of applications.
The present invention overcomes each of the prior art inadequacies and is adaptable to a broader range of applications with a greater quality of performance.